JPH03119311A - Optical modulator - Google Patents

Abstract

PURPOSE:To obtain the optical modulator suitable for high-speed and low-power consumption operations by forming a striped optical waveguide in a semiconductor waveguide layer and impressing an electric field to the semiconductor waveguide layer in the three-dimensional optical waveguide part. CONSTITUTION:A 1st semiconductor clad layer 102, a semiconductor layer 103 and a 2nd semiconductor clad layer 104 are laminated on a (111) semiconductor substrate 101. Light is confined in a three-dimensional, i.e. direction perpendicular to the progressing direction of the light in the p<->-GaAs waveguide layer 103 right under a rib and the light is propagated along the striped rib part. The refractive indices of the p<->-GaAs waveguide layer 103 and the p<->-AlGaAs clad layer 104 are changed by the primary electrooptical effect when the reverse bias voltage is impressed between (p) side and (n) side electrodes at this time. The lights confined in the p<->-GaAs waveguide layer 103 and the p<->-AlGaAs clad layer 104 are subjected to a phase change. The optical modulator suitable for the high-speed and low-power consumption operations is obtd. in this way.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to a semiconductor optical modulator that will be an important element in future high-speed optical communication systems, and in particular to a high-efficiency optical modulator using a compound semiconductor as a material. .

[Conventional technology]

Optical modulators are considered to be one of the key elements of future high-speed optical communication systems, and research and development are becoming active in various places. Optical modulators are thought to be made of dielectric materials such as LiNb03, or semiconductors such as GaAa or InP, but expectations are high for semiconductor optical modulators that are compact and capable of highly efficient operation. It has been increasing in recent years. Considering the above application fields, such a semiconductor optical modulator is required to operate at high speed, with low power consumption, and at low voltage. The physical effects needed to realize such a high-speed optical modulator include the first-order electro-optic effect (which operates at high speed in principle).
Pockels effect) is commonly used. In this first-order electro-optic effect, when an electric field is applied to a medium, its refractive index increases or decreases linearly with respect to the applied electric field.

Therefore, G, which is a medium with a first-order electro-optic effect,
A three-dimensional optical waveguide, that is, an optical waveguide with an optical confinement structure in the surface direction perpendicular to the direction of light propagation, is formed on an As-based or InP-based compound semiconductor, and an electric field is effectively applied to the waveguide layer. If applied, a phase change due to a change in refractive index can be imparted to propagating light. Therefore, optical phase modulators, directional coupling optical modulators, branching interference optical modulators, etc. can be realized using this principle.

[Problem to be solved by the invention]

By the way, the lower the operating voltage of these optical modulators is, the more desirable it is for high-speed operation and low power consumption operation.
Furthermore, the shorter the element length, the smaller the electrode capacitance, which is desirable for speeding up. In order to reduce the operating voltage and element length of such an optical modulator, it is necessary that the amount of change in the refractive index be as large as possible. the current,
As the above-mentioned material for the optical modulator, Ga in the (100) direction is used.
Although As yarn materials and InP-based materials are being considered,
The amount of change in refractive index per unit voltage/unit length in this orientation is smaller than that in the (111) orientation, and it cannot be said that the characteristics of the material are utilized to the fullest.

[Means for solving problems]

In order to solve the above problems, in the present invention, at least a first semiconductor cladding layer, a semiconductor waveguide layer, and a second semiconductor cladding layer are laminated on a (111) semiconductor substrate, A structure of an optical modulator comprising means for forming a striped optical waveguide in a semiconductor waveguide layer, and means for applying an electric field to the semiconductor waveguide layer of the three-dimensional optical waveguide portion, or (111) At least a first semiconductor cladding layer, a semiconductor waveguide layer, and a second semiconductor cladding layer are laminated on the semiconductor substrate], and two adjacent striped optical waveguides are formed in the conductive waveguide layer. means and
A structure of a directional coupler type optical modulator comprising the two striped optical waveguides and a means for forming an optical combiner, and (111) a first semiconductor on a semiconductor substrate. At least a cladding layer, a semiconductor waveguide layer, and a second semiconductor cladding layer are laminated, and on the semiconductor substrate,
A 3a optical splitter having a three-dimensional optical waveguide structure, two phase modulators, means for forming an optical combiner, and a means for forming the optical phase modulator and optical combiner of 2'). The structure of the branching interference optical modulator is adopted.
□ [Function] In the present invention, the (111) orientation is used as the crystal, and the TM mode is used as the polarization state of the incident light.
The amount of change in refractive index (unit voltage - unit length) is larger than that when the (100) direction is used and the TE mode is used as the incident light. Specifically, in a zinc blende crystal such as GaAs+InP, the absolute value of the refractive index change Δn when the (100) oriented crystal is used and the polarization state of the incident light is TE mode is n6. Waveguide effective refractive index, r41
When is the electro-optic coefficient and E is the electric field strength, it is expressed as 1Δn l = "n: r41 B. On the other hand, when the 1Δnl=
"n: r41E 1-", so using the s (111) orientation TM mode has the effect of reducing the voltage by a factor of two, or for the same operating voltage. When the polarization state of the light is TM mode, no refractive index change occurs at all.In addition, in the case of (111) orientation TE mode, the absolute value of the refractive index change amount Δn is 2 1Δ"' : 2d nO"" So! ? , (100) azimuth TE mode] is also smaller.

〔Example〕

The present invention will be described in detail below with reference to the drawings.

FIG. 1 shows GaAs/Al! according to the present invention. FIG. 2 is a perspective view showing an example of a GaAs optical phase modulator. In the figure, a Lip type GaAs/AIG aAs optical phase modulator formed on a (111) n+-GaAs substrate 101 is shown.

First, a method for manufacturing the GaAs/AlGaAs optical phase modulator shown in FIG. 1 will be briefly described. n-A I G on the (111)-oriented n+-GaAs substrate 101.
a As (Al composition ratio X=α5) cladding layer 102
L is about 5pm, p--GaAs waveguide layer 103 is α3μ
The m1p-AIGaAs (kl composition ratio X=α5) cladding layer 104 is made of α1 fin, p-AlGaAs (
Composition ratio of Al x = 0.5) cladding layer 108 is 1.0
μff1%p” GaAs cap layer 105 is o, zμ
m, MBE (Mo-Icular Beam
They are sequentially laminated using the epitaxy method. after that,
After depositing a Ti/Au film, which will become the p-side electrode 106, on the entire surface of the substrate, this Ti/Au film is
Process the u film into a stripe shape. Furthermore, the Ti/Au film processed into this stripe shape and the Ti/Au
RIBE using the photoresist left on the film as a mask
(Reactive Ion-Beam Etcb
ing) method, the parts other than the stripe area are coated with p-Al.
Etching is performed until the interface between the GaAs cladding layer 108 and the p--AlGaAs cladding layer 104 is reached, thereby forming a lip type optical waveguide as shown in FIG. So,
After (111) polishing of the n-GaAs substrate 100, n
A u G e N i /A u which becomes the side electrode 107
After performing Ni vapor deposition and electrode alloying, the device is divided into two lengths to complete the fabrication of the optical modulator. Here, the width of the lip type optical waveguide is 2 to 5 μm.

Next, the principle of operation of the GaAs/AlGaAs optical phase modulator shown in FIG. 1 will be explained. In the optical phase modulator with the structure shown in FIG.
The light is confined in the waveguide layer 103 three-dimensionally, that is, in the direction perpendicular to the direction in which the light travels, and the light propagates along the striped rib portions. At this time, when a reverse bias voltage is applied between the p-side and n@ electrodes, 1) the pGaAs waveguide layer 103 and the p--A/G
The refractive index of the aAs cladding layer 104 changes, and p--
In the GaAs waveguide layer 103 and p--A I Ga
The light confined in the As cladding layer 104 undergoes a phase change. In the present invention, the crystal orientation is (11
1), since the TM mode is used as the incident light polarization state, the difference between the phase change amount Δβ and the refractive index change amount Δ per unit length is “ ” n”r E−r Δβ=k・△n ＝end×v'To4'' 2π I``3 v''()'τ'Or41 h λ (k: wave number h: p-layer thickness) (r: confinement coefficient ■: applied voltage) relationship is The surface voltage required to change the phase by π, that is, the half-wave voltage, is expressed as follows. In the above equation, wavelength λ = 1.3μfn, p-layer thickness h = 0.4 am, optical confinement coefficient r = 0.8
, effective refractive index n. =3.22, electro-optic coefficient '41 =
: 1.6 x 10-” (m/V), element length L=2
By substituting 11, we obtain 5.3v as the half-wave voltage.

The value of this half-wave voltage is approximately 15
% reduced. The element capacitance of this optical phase modulator is estimated to be Ll pF from the depletion layer capacitance, and the modulation frequency band is expected to be approximately 6 GHz.

FIG. 2 is a perspective view showing an embodiment of a GaAs/AjGaAs directional coupler type optical modulator according to the present invention.

In the figure, lip type GaAs/Al! formed on a (111)n-GaAs substrate 101! A GaAs directional coupler type optical modulator is shown.

First, a method for manufacturing the GaAs/AjGaAs directional coupler type optical modulator shown in FIG. 2 will be briefly described. (1
11) On the n''-GaAs substrate 101 in the orientation, a p--GaAs waveguide layer 1 is formed with an n-GaAs (composition ratio x = 0.5) layer 102 of about 1.5m.
03 is 0.3 μm, p--A/GaAs (At composition ratio !==0.5) cladding layer 104 is 0.1 μm, p-A
/GaAs (composition ratio of A1 x = 0.5) cladding layer 10B is 1.0 Am, p"-GaAs) Yayp layer 10
5 at 0.2flm, MBE (Molecular Be
am epitaxy) method. Thereafter, a Ti/Au film that will become the p-side electrodes 106a and 106b is deposited over the entire surface of the substrate, and then this Ti/Au film is processed into two stripes using a normal photolithography method. Furthermore, Tt/Au processed into this stripe shape
The photoresist left on the film and the Tt/Au film was masked off and subjected to RIBE (Reactive IonBea).
The p-AjGaAs cladding layer 108 and the p--AzG
The aAs cladding layer 104 is removed by etching until the layer surface 04 is reached, and a lip type optical waveguide as shown in FIG.
Books, form close together.

After that, polishing of the (111)n''-GaAa substrate 100, n
After vapor deposition of AuGeNi/AuNi that will become the side electrode 107 and electrode alloying, the device is cleaved to a length of 2 fl to complete the device fabrication. ζHere, the width of the optical waveguide is 25 μm, and the interval between the waveguides is 25 μm.

Next, the operating principle of the GaAs/A4GaAs directional coupler type optical modulator shown in FIG. 2 will be explained. In the directional coupler type optical modulator having the structure shown in FIG. Light propagates along the waveguide, but since the distance between the two optical waveguides is small, coupling occurs between the two waveguides. Therefore, light incident on one optical waveguide is completely coupled to the other optical waveguide after propagating a certain length. This length is called the fully bonded length. Here, the element length (2 lines) is selected to be this perfect bond length. Therefore, when no reverse bias voltage is applied, the light input to the negative optical waveguide is emitted from the other optical waveguide. On the contrary,
When a reverse bias voltage is applied to only one of the two optical waveguides, the refractive index of that optical waveguide changes due to the electro-optic effect, and the phase matching condition between the two leading waveguides is disrupted. When the phase difference between the two optical waveguides becomes 0π, coupling no longer occurs between the 12 optical waveguides, and the light incident from one optical waveguide exits from the same optical waveguide, causing light switching. . Here, since the voltage Vs required for 100% modulation is the voltage for giving a phase change of dπ, calculations similar to those in the first embodiment can be performed.
,=12V. In this case as well, the voltage required for modulation is approximately 15% compared to the case where the (Zoo) azimuth TE mode is used.
reduced. Note that the element capacitance of this directional coupler type modulator is also estimated to be ttpF from the depletion layer capacitance as in the first embodiment, and a modulation frequency band of approximately 5 GHz' is expected.

FIG. 3 is a perspective view showing an embodiment of the GaAs/AjGaAs branching interference type optical modulator according to the present invention. In FIG. s/person/GaAs interferometric optical modulator is shown.

The layer structure of the GaAs/AjGaAs branch interference optical modulator shown in FIG. are the same. Note that in this embodiment, as shown in FIG.
A slit 304 is provided at the boundary of 01 and the boundary of the phase modulator and merging portion 303, respectively. Here, the width of the optical waveguide is 2 to 5 μm, and the length of the two linear phase modulators is 2M.

In Fig. 3, the TM input into the optical waveguide on the input side
The mode light passes through two optical waveguides (
The signal is branched 1:1 to the phase modulator section 302a, b). 2
When no reverse bias voltage is applied to either of the two phase modulators 302a, 302b, the lights branched into the two optical waveguides are combined in the same phase at the merging section 303, and are emitted as they are from the output side optical waveguide. When a reverse bias voltage is applied to one of the two phase modulators, the phase of the light propagating through the optical waveguide to which the reverse bias voltage is applied changes due to the electro-optic effect, and the amount of this phase change is If it is π, the lights with opposite phases are merged at the merge section 303, so in this case the guided light is radiated into the substrate,
No light is emitted from the output side optical waveguide. therefore,
It operates as an optical modulator capable of 0N10FF depending on voltage. In the case of this branching interference type optical modulator, the voltage required for 100% modulation is the same as the half-wave voltage of the optical phase modulator shown in the first embodiment, so the voltage required for wavelength LSI is 5.3V.
The reduction is approximately 15% compared to the case where the (100) azimuth TE mode is used. In addition, the element capacitance is also the same as that of the phase modulator shown in the first embodiment, and is 1 from the depletion layer capacitance.
．． estimated to be 1F, with a modulation frequency band of approximately 5 GHz
There is expected.

〔Effect of the invention〕

As described above, according to the present invention, the normal (100)
Compared to the case of using the azimuth TE mode, the refractive index change per unit voltage/unit length can be increased. Therefore, according to the present invention, an optical modulator with the same element length can have a lower operating voltage than an optical modulator with a (100) orientation, and when the same operating voltage is allowed, Since the length of the moving element can be shortened, the element capacitance can be reduced.

Therefore, it is possible to provide an optical modulator suitable for high-speed, low-power operation, which will greatly contribute to the field of high-speed optical communications.

Note that the present invention is not limited to the above embodiments. Although a GaAs-based optical modulator is used as an example, the present invention is not limited to this (the present invention is also applicable to optical modulators using other materials such as InP-based). .

In addition, although the optical waveguide structure for realizing the modulator and the lip type optical waveguide are taken as an example in the embodiments, the present invention is not limited to this, and other structures such as a buried type may also be used. It goes without saying that the device shape shown in the example is not limited to the thickness of each layer, the composition of each layer, the waveguide dimensions, etc.

[Brief explanation of drawings]

FIG. 1 shows a first embodiment of the present invention.
/A perspective view showing the structure of an AlGa As optical phase modulator,
FIG. 2 shows a second embodiment of the present invention, GaAs/Aj
Figure 3 showing the structure of a GaAs directional coupler type optical modulator.
The figure shows the third embodiment of the present invention, GaAs/AI! Ga
FIG. 2 is a diagram showing the structure of an As branching interference type optical modulator. 101- (111) n”-GaAs substrate, 102
= n-Al! GaAs cladding layer, 103-
p--GaAs waveguide layer, 104...-p--AjGa
As cladding layer, 105-...p''-G a A
s'r7 skip layer, 106a, 106b-p side electrode, 10
7-n side electrode, 108-p-AjGaAs cladding layer, 301.3 dB branch, 302a, 302b...
Phase modulator section, 303... merging section, 304... slit. party 1 mouth

Claims (3)

[Claims] (1) (111) At least a first semiconductor cladding layer, a semiconductor waveguide layer, and a second semiconductor cladding layer are laminated on a semiconductor substrate, and a striped optical waveguide is formed in the semiconductor waveguide layer. and means for applying an electric field to the semiconductor waveguide layer of the striped optical waveguide portion. (2) (111) At least a first semiconductor cladding layer, a semiconductor waveguide layer, and a second semiconductor cladding layer are laminated on a semiconductor substrate, and two adjacent striped optical waveguides are connected to the semiconductor waveguide. An optical modulator comprising: means formed in a layer; and means for independently applying an electric field to each of the two striped optical waveguides. (3) (111) At least a first semiconductor cladding layer, a semiconductor waveguide layer, and a second semiconductor cladding layer are laminated on a semiconductor substrate, and a first semiconductor cladding layer, a semiconductor waveguide layer, and a second semiconductor cladding layer are stacked on the semiconductor substrate in a direction perpendicular to the traveling direction of light. means for forming a 3 dB optical splitter, two phase modulators, and an optical combiner made of an optical waveguide structure having an optical confinement effect; and means for independently applying an electric field to each of the two optical phase modulators. An optical modulator comprising: